MCQ QUESTION BANK Design of DC Machine and Transformer 1

Faculty of Degree Engineering - 083 Department of EC / Electrical Engineering -09

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Dr. Subhash Technical Campus- The Jewel of Junagadh

Design of DC Machine and Transformer – 1

Subject code: -2160912 1. Which of the following does not change in a transformer? (a) Current (b) Voltage (c) Frequency (d) All of the above 2. In a transformer the energy is conveyed from primary to secondary (a) through cooling coil (b) through air (c) by the flux (d) none of the above 3. A transformer core is laminated to (a) reduce hysteresis loss (b) reduce eddy current losses (c) reduce copper losses (d) reduce all above losses 4. The degree of mechanical vibrations produced by the laminations of a transformer depends on (a) tightness of clamping (b) gauge of laminations (c) size of laminations (d) all of the above Ans: 1.c 2.C 3. B 4.D


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5. The noload current drawn by transformer is usuallywhat per cent of the fullload current ? (a) 0.2 to 0.5 per cent (b) 2 to 5 per cent (c) 12 to 15 per cent (d) 20 to 30 per cent 6. The path of a magnetic flux in a transformer should have (a) high resistance (b) high reluctance (c) low resistance (d) low reluctance Ans: d 7. Noload on a transformer is carried out to determine (a) copper loss (b) magnetising current (c) magnetising current and loss (d) efficiency of the transformer Ans: c 8. The dielectric strength of transformer oil is expected to be (a) lkV (b) 33 kV (c) 100 kV (d) 330 kV Ans: b 9. Sumpner’s test is conducted on transformers to determine (a) temperature (b) stray losses (c) allday efficiency (d) none of the above Ans: a Ans:5. B 6.D 7.C 8.B 9.A


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10. The permissible flux density in case of cold rolled grain oriented steel is around (a) 1.7 Wb/m2 (b) 2.7 Wb/m2 (c) 3.7 Wb/m2 (d) 4.7 Wb/m2 11. The efficiency of a transformer will be maximum when (a) copper losses = hysteresis losses (b) hysteresis losses = eddy current losses (c) eddy current losses = copper losses (d) copper losses = iron losses 12. Noload current in a transformer (a) lags behind the voltage by about 75° (b) leads the voltage by about 75° (c) lags behind the voltage by about 15° (d) leads the voltage by about 15° 13. The purpose of providing an iron core in a transformer is to (a) provide support to windings (b) reduce hysteresis loss (c) decrease the reluctance of the magnetic path (d) reduce eddy current losses 14. Which of the following is not a part of transformer installation ? (a) Conservator (b) Breather (c) Buchholz relay (d) Exciter Ans: 10.A 11.D 12.A 13.C 14.D


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15. While conducting shortcircuit test on a transformer the following side is short circuited (a) High voltage side (b) Low voltage side (c) Primary side (d) Secondary side 16. In the transformer following winding has got more crosssectional area (a) Low voltage winding (b) High voltage winding (c) Primary winding (d) Secondary winding 17. A transformer transforms (a) voltage (b) current (c) power (d) frequency 18. A transformer cannot raise or lower the voltage of a D.C. supply because (a) there is no need to change the D.C. voltage (b) a D.C. circuit has more losses (c) Faraday’s laws of electromagnetic induction are not valid since the rate of change of flux is zero (d) none of the above 19. Primary winding of a transformer (a) is always a low voltage winding (b) is always a high voltage winding (c) could either be a low voltage or high voltage winding (d) none of the above

ANS 15.B 16.A 17.C 18.C 19.C


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20. Which winding in a transformer has more number of turns ? (a) Low voltage winding (b) High voltage winding (c) Primary winding (d) Secondary winding 21. Efficiency of a power transformer is of the order of (a) 100 per cent (b) 98 per cent (c) 50 per cent (d) 25 per cent 22. In a given transformer for given applied voltage, losses which remain constant irrespective of load changes are (a) friction and windage losses (b) copper losses (c) hysteresis and eddy current losses (d) none of the above 23. A common method of cooling a power transformer is (a) natural air cooling (b) air blast cooling (c) oil cooling (d) any of the above 24. The no load current in a transformer lags behind the applied voltage by an angle of about (a) 180° (b) 120″ (c) 90° (d) 75° ANS 20 B21C 22C 23C 24D


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25. In a transformer routine efficiency depends upon (a) supply frequency (b) load current (c) power factor of load (d) both (b) and (c) 26. In the transformer the function of a conservator is to (a) provide fresh air for cooling the transformer (b) supply cooling oil to transformer in time of need (c) protect the transformer from damage when oil expends due to heating (d) none of the above 27. Natural oil cooling is used for transformers up to a rating of (a) 3000 kVA (b) 1000 kVA (c) 500 kVA (d) 250 kVA 28. Power transformers are designed to have maximum efficiency at (a) nearly full load (b) 70% full load (c) 50% full load (d) no load 29. The maximum efficiency of a distribution transformer is (a) at no load (b) at 50% full load (c) at 80% full load (d) at full load 30. Transformer breaths in when (a) load on it increases (b) load on it decreases (c) load remains constant (d) none of the above Ans 25,26,27,28,29,30 d,c,a,a,b,b


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31. Noload current of a transformer has (a) has high magnitude and low power factor (b) has high magnitude and high power factor (c) has small magnitude and high power factor (d) has small magnitude and low power factor 32. Spacers are provided between adjacent coils (a) to provide free passage to the cooling oil (b) to insulate the coils from each other (c) both (a) and (b) (d) none of the above 33. Greater the secondary leakage flux (a) less will be the secondary induced e.m.f. (b) less will be the primary induced e.m.f. (c) less will be the primary terminal voltage (d) none of the above 34. The purpose of providing iron core in a stepup transformer is (a) to provide coupling between primary and secondary (b) to increase the magnitude of mutual flux (c) to decrease the magnitude of magnetizing current (d) to provide all above features 35. The power transformer is a constant (a) voltage device (b) current device (c) power device (d) main flux device 36. Two transformers operating in parallel will share the load depending upon their (a) leakage reactance (b) per unit impedance (c) efficiencies (d) ratings Ans:d,a,a,c,d,d


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37. If R2 is the resistance of secondary winding of the transformer and K is the transformation ratio then the equivalent secondary resistance referred to primary will be (a) R2/VK (b) R2IK2 (c) R22!K2 (d) R22/K Ans: b 38. What will happen if the transformers working in parallel are not connected with regard to polarity ? (a) The power factor of the two transformers will be different from the power factor of common load (b) Incorrect polarity will result in dead short circuit (c) The transformers will not share load in proportion to their kVA ratings (d) none of the above Ans: b 39. If the percentage impedances of the two transformers working in parallel are different, then (a) transformers will be overheated (b) power factors of both the transformers will be same (c) parallel operation will be not possible (d) parallel operation will still be possible, but the power factors at which the two transformers operate will be different from the power factor of the common load Ans: d 40. In a transformer the tappings are generally provided on (a) primary side (b) secondary side (c) low voltage side (d) high voltage side Ans: c 41. The use of higher flux density in the transformer design (a) reduces weight per kVA ( B) reduces iron losses (c) reduces copper losses (d) increases part load efficiency Ans: a


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42. The chemical used in breather for transformer should have the quality of (a) ionizing air (b) absorbing moisture (c) cleansing the transformer oil (d) cooling the transformer oil. Ans: b 43. The chemical used in breather is (a) asbestos fiber (b) silica sand (c) sodium chloride (d) silica gel Ans: d 44. An ideal transformer has infinite values of primary and secondary inductances. The statement is (a) true (b) false Ans: b 45. The transformer ratings are usually expressed in terms of (a) volts (b) amperes (c) kW (d) kVA Ans: d 46. The noise resulting from vibrations of laminations set by magnetic forces, is termed as (a) magnetostrication (b) boo (c) hum (d) zoom Ans: c 47. Hysteresis loss in a transformer varies as CBmax = maximum flux density) (a) Bmax (b) Bmax16 (C) Bmax183 Ans: b


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48. Material used for construction of transformer core is usually (a) wood (b) copper (c) aluminium (d) silicon steel 49. The thickness of laminations used in a transformer is usually (a) 0.4 mm to 0.5 mm (b) 4 mm to 5 mm (c) 14 mm to 15 mm (d) 25 mm to 40 mm 50. The function of conservator in a transformer is (a) to project against’internal fault (b) to reduce copper as well as core losses (c) to cool the transformer oil (d) to take care of the expansion and contraction of transformer oil due to variation of temperature of sur roundings 51. The highest voltage for transmitting electrical power in India is (a) 33 kV. (6) 66 kV (c) 132 kV (d) 400 kV 52. In a transformer the resistance between its primary and secondary is (a) zero (b) 1 ohm (c) 1000 ohms (d) infinite 53. A transformer oil must be free from (a) sludge (b) odour (c) gases (d) moisture Ans d,a,d,d,d,d


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54. A Buchholz relay can be installed on (a) autotransformers (b) aircooled transformers (c) welding transformers (d) oil cooled transformers Ans: d 55. Gas is usually not liberated due to dissociation of transformer oil unless the oil temperature exceeds (a) 50°C (b) 80°C (c) 100°C (d) 150°C Ans: d 56. The main reason for generation of harmonics in a transformer could be (a) fluctuating load (b) poor insulation (c) mechanical vibrations (d) saturation of core Ans: d 57. Distribution transformers are generally designed for maximum efficiency around (a) 90% load (b) zero load (c) 25% load (d) 50% load Ans: d 58. Which of the following property is not necessarily desirable in the material for transformer core ? (a) Mechanical strength (6) Low hysteresis loss (c) High thermal conductivity (d) High permeability Ans: c


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59. Star/star transformers work satisfactorily when (a) load is unbalanced only (b) load is balanced only (c) on balanced as well as unbalanced loads (d) none of the above 60. Delta/star transformer works satisfactorily when (a) load is balanced only (b) load is unbalanced only (c) on balanced as well as unbalanced loads (d) none of the above 61. Buchholz’s relay gives warning and protection against (a) electrical fault inside the transformer itself (b) electrical fault outside the transformer in outgoing feeder (c) for both outside and inside faults (d) none of the above 62. The magnetising current of a transformer is usually small because it has (a) small air gap (b) large leakage flux (c) laminated silicon steel core (d) fewer rotating parts 63. Which of the following does not change in an ordinary transformer ? (a) Frequency (b) Voltage (c) Current (d) Any of the above 64. Which of the following properties is not necessarily desirable for the material for transformer core ? (a) Low hysteresis loss (b) High permeability (c) High thermal conductivity (d) Adequate mechanical strength Ans: b,c,a,a,a,c


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65. The leakage flux in a transformer depends upon (a) load current (b) load current and voltage (c) load current, voltage and frequency (d) load current, voltage, frequency and power factor 66. The path of the magnetic flux in transformer should have (a) high reluctance (b) low reactance (c) high resistance (d) low resistance 67. Noise level test in a transformer is a (a) special test (b) routine test (c) type test (d) none of the above 68. Which of the following is not a routine test on transformers ? (a) Core insulation voltage test (b) Impedance test (c) Radio interference test (d) Polarity test 69. A transformer can have zero voltage regulation at (a) leading power factor (b) lagging power factor (c) unity power factor (d) zero power factor 70. Helical coils can be used on (a) low voltage side of high kVA transformers (b) high frequency transformers (c) high voltage side of small capacity transformers (d) high voltage side of high kVA rating transformers Ans: a,b,c,c,a,a


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71. Harmonics in transformer result in (a) increased core losses (b) increased I2R losses (c) magnetic interference with communication circuits (d) all of the above 72. The core used in high frequency transformer is usually (a) copper core (b) cost iron core (c) air core (d) mild steel core 73. The fullload copper loss of a trans¬former is 1600 W. At halfload, the copper loss will be (a) 6400 W (b) 1600 W (c) 800 W (d) 400 W 74. The value of flux involved m the e.m.f. equation of a transformer is (a) average value (b) r.m.s. value (c) maximum value (d) instantaneous value 75. Silicon steel used in laminations mainly reduces (a) hysteresis loss (b) eddy current losses (c) copper losses (d) all of the above 76. Which winding of the transformer has less crosssectional area ? (a) Primary winding (b) Secondary winding (c) Low voltage winding (d) High voltage winding Ans: d,c,d,c,a,d


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77. Power transformers are generally designed to have maximum efficiency around (a) noload (b) halfload (c) near fullload (d) 10% overload Ans: c 78. Which of the following is the main advantage of an autotransformer over a two winding transformer ? (a) Hysteresis losses are reduced (b) Saving in winding material (c) Copper losses are negligible (d) Eddy losses are totally eliminated Ans: b 79. During short circuit test iron losses are negligible because (a) the current on secondary side is negligible (b) the voltage on secondary side does not vary (c) the voltage applied on primary side is low (d) fullload current is not supplied to the transformer Ans: c 80. Two transformers are connected in parallel. These transformers do not have equal percentage impedance. This is likely to result in (a) shortcircuiting of the secondaries (b) power factor of one of the transformers is leading while that of the other lagging (c) transformers having higher copper losses will have negligible core losses (d) loading of the transformers not in proportion to their kVA ratings Ans: d 81. The changes in volume of transformer cooling oil due to variation of atmospheric temperature during day and night is taken care of by which part of transformer (a) Conservator (b) Breather (c) Bushings (d) Buchholz relay Ans: a


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82. An ideal transformer is one which has (a) no losses and magnetic leakage (b) interleaved primary and secondary windings (c) a common core for its primary and secondary windings (d) core of stainless steel and winding of pure copper metal (e) none of the above 83. When a given transformer is run at its rated voltage but reduced frequency, its (a) flux density remains unaffected (b) iron losses are reduced (c) core flux density is reduced (d) core flux density is increased 84. In an actual transformer the iron loss remains practically constant from noload to fullload because (a) value of transformation ratio remains constant (b) permeability of transformer core remains constant (c) core flux remains practically constant (d) primary voltage remains constant (c) secondary voltage remains constant 85. An ideal transformer will have maximum efficiency at a load such that (a) copper loss = iron loss (b) copper loss < iron loss (c) copper loss > iron loss (d) none of the above 86. If the supply frequency to the transformer is increased,”the iron loss will (a) not change (b) decrease (c) increase (d) any of the above 87. Negative voltage regulation is indicative that the load is (a) capacitive only (b) inductive only (c) inductive or resistive (d) none of the above Ans: a,d,c,a,c,a


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88. Iron loss of a transformer can be measured by (a) low power factor wattmeter (b) unity power factor wattmeter (c) frequency meter (d) any type of wattmeter 89. When secondary of a current transformer is opencircuited its iron core will be (a) hot because of heavy iron losses taking place in it due to high flux density (b) hot because primary will carry heavy current (c) cool as there is no secondary current (d) none of above will happen 90. The transformer laminations are insulated from each other by (a) mica strip (6) thin coat of varnish (c) paper (d) any of the above 91. Which type of winding is used in 3phase shelltype transformer ? (a) Circular type (b) Sandwich type (c) Cylindrical type (d) Rectangular type 92. During open circuit test of a transformer (a) primary is supplied rated voltage (b) primary is supplied fullload current (c) primary is supplied current at reduced voltage (d) primary is supplied rated kVA 93. Open circuit test on transformers is conducted to determine (a) hysteresis losses (b) copper losses (c) core losses (d) eddy current losses Ans: a,a,b,b,a,c


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94. Short circuit test on transformers is conducted to determine (a) hysteresis losses (b) copper losses (c) core losses (d) eddy current losses 95. For the parallel operation of single phase transformers it is necessary that they should have (a) same efficiency (b) same polarity (c) same kVA rating (d) same number of turns on the secondary side. 96. The transformer oil should have volatility and viscosity. (a) low,low (b) high,high (c) low,high (d) high,low 97. The function of breather in a transformer is (a) to provide oxygen inside the tank (b) to cool the coils during reduced load (c) to cool the transformer oil (d) to arrest flow of moisture when outside air enters the transformer 98. The secondary winding of which of the following transformers is always kept closed ? (a) Stepup transformer (b) Stepdown transformer (c) Potential transformer (d) Current transformer 99. The size of a transformer core will depend on (a) frequency (b) area of the core (c) flux density of the core material (d) (a) and (b) both Ans: b,b,a,d,d,d


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100. N atural air coo ling is generally restricted for transformers up to (a) 1.5 MVA (b) 5 MVA (c) 15 MVA (d) 50 MVA 101. A shelltype transformer has (a) high eddy current losses (b) reduced magnetic leakage (c) negligibly hysteresis losses (d) none of the above 102. A transformer can have regulation closer to zero (a) on fullload (b) on overload (c) on leading power factor (d) on zero power factor 103. A transformer transforms (a) voltage (b) current (c) current and voltage (d) power 104. Which of the following is not the standard voltage for power supply in India ? (a) llkV (b) 33kV (c) 66 kV (d) 122 kV 105. Reduction in core losses and increase in permeability are obtained with transformer employing (a) core builtup of laminations of cold rolled grain oriented steel (b) core builtup of laminations of hot rolled sheet (c) either of the above (d) none of the above Ans: a,b,c,d,d,a


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106. In a power or distribution transformer about 10 per cent end turns are heavily insulated (a) to withstand the high voltage drop due to line surge produced by the shunting capacitance of the end turns (b) to absorb the line surge voltage and save the winding of transformer from damage (c) to reflect the line surge and save the winding of a transformer from damage (d) none of the above 107. For given applied voltage, with the increase in frequency of the applied voltage (a) eddy current loss will decrease (b) eddy current loss will increase (c) eddy current loss will remain unchanged (d) none of the above 108. Losses which occur in rotating electric machines and do not occur in trans formers are (a) friction and windage losses (b) magnetic losses (c) hysteresis and eddy current losses (d) copper losses 109. In a given transformer for a given applied voltage, losses which remain constant irrespective of load changes are (a) hysteresis and eddy current losses (b) friction and windage losses (c) copper losses (d) none of the above 110. Which of the following statements regarding an idel singlephase transformer having a turn ratio of 1 : 2 and drawing a current of 10 A from 200 V A.C. supply is incorrect ? (a) Its secondary current is 5 A (b) Its secondary voltage is 400 V (c) Its rating is 2 kVA (d) Its secondary current is 20 A (e) It is a stepup transformer 111. The secondary of a current transformer is always shortcircuited under operating conditions because it (a) avoids core saturation and high voltage induction (b) is safe to human beings (c) protects the primary circuit (d) none of the above Ans: a,c,a,a,d,a


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112. In a transformer the resistance between its primary and secondary should be (a) zero (b) 10 Q (c) 1000 Q (d) infinity 113. A good voltage regulation of a transformer means (a) output voltage fluctuation from no load to full load is least (b) output voltage fluctuation with power factor is least (c) difference between primary and secondary voltage is least (d) difference between primary and secondary voltage is maximum 114. For a transformer, operating at constant load current, maximum efficiency will occur at (a) 0.8 leading power factor (b) 0.8 lagging power factor (c) zero power factor (d) unity power factor 115. Which of the following protection is normally not provided on small distribution transformers ? (a) Overfluxing protection (b) Buchholz relay (c) Overcurrent protection (d) All of the above 116. Which of the following acts as a protection against high voltage surges due to lightning and switching ? (a) Horn gaps (b) Thermal overload relays (c) Breather 117. The efficiency of two identical transformers under load conditions can be determined by (a) shortcircuit test (b) backtoback test (c) open circuit test Ans: d,a,d,b,a,b


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118. Which of the following insulating materials can withstand the highest temperature safely ? (a) Cellulose (b) Asbestos (c) Mica (d) Glass fibre 119. Which of the following parts of a transformer is visible from outside ? (a) Bushings (b) Core (c) Primary winding (d) Secondary winding 120. The noise produced by a transformer is termed as (a) zoom (b) hum (c) ringing (d) buzz 121. Which of the following loss in a transformer is zero even at full load ? (a) Core loss (b) Friction loss (c) Eddy current loss (d) Hysteresis loss 122. Which of the following is the most likely source of harmonics in a transformer ? (a) poor insulation (b) Overload (c) loose connections (d) Core saturation 123. If a transformer is continuously operated the maximum temperature rise will occur in (a) core (b) windings (c) tank (d) any of the above Ans: c,a,b,b,d,b


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124. The hum in a transformer is mainly attributed to (a) load changes (b) oil in the transformer (c) magnetostriction (d) mechanical vibrations 125. The maximum load that a power transformer can carry is limited by its (a) temperature rise (b) dielectric strength of oil (c) voltage ratio (d) copper loss 126. The efficiency of a transformer, under heavy loads, is comparatively low because (a) copper loss becomes high in proportion to the output (b) iron loss is increased considerably (c) voltage drop both in primary and secondary becomes large (d) secondary output is much less as compared to primary input 127. An opencircuit test on a transformer is conducted primarily to measure (a) insulation resistance (b) copper loss (c) core loss (d) total loss (e) efficiency (f) none of the above 128. A noload test is performed on a transformer to determine (a) core loss (b) copper loss (c) efficiency (d) magnetising current (e) magnetising current and loss 129. The voltage transformation ratio of a transformer is equal to the ratio of (a) primary turns to secondary turns (b) secondary current to primary current (c) secondary induced e.m.f. to primary induced e.m.f. (d) secondary terminal voltage to primary applied voltage Ans: c,c,a,c,e,c


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130. Part of the transformer which is most subject to damage from overheating is (a) iron core (b) copper winding (c) winding insulation (d) frame or case (e) transformer tank 131. If a transformer is switched on to a voltage more than the rated voltage (a) its power factor will deteriorate (b) its power factor will increase (c) its power factor will remain unaffected (d) its power factor will be zero 132. Autotransformer makes effective saving on copper and copper losses, when its transformation ratio is (a) approximately equal to one (b) less than one (c) great than one (d) none of the above 133. Minimum voltage regulation occurs when the power factor of the load is (a) unity (b) lagging (c) leading (d) zero 134. In a stepdown transformer, there is a change of 15 A in the load current. This results in change of supply current of (a) less than 15 A (b) more than 15 A (c) 15 A Ans: a 135. The efficiencies of transformers compared with that of electric motors of the same power are (a) about the same (6) much smaller (c) much higher Ans: c,a,a,c,a,c


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136 . Which of the following is the major consideration to evolve a good design ? (a) Cost (b) Durability (c) Compliance with performance criteria as laid down in specifications (d) All of the above 137. impose limitation on design. (a) Saturation (b) Temperature rise (c) Efficiency (d) Power factor (e) All above 138. The efficiency of a machine should be as ______ as possible to reduce the operating cost. (a) high (b) low (c) either of the above 139. If an insulating material is operated beyond the maximum allowable temperature, its life is (a) drastically increased (b) drastically reduced (c) unaffected (d) none of the above 140. The design of mechanical parts is particularly important in case of _____ speed machines. (a) low (b) medium (c) high 141. In induction motors, the length of air gap in kept as small as mechanically possible is order to have (a) low power factor (b) high power factor (c) high over load capacity Ans: d,e,a,b,c,b


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142. In ______ machines, the size of the shaft is decided by the critical speed which depends on the deflection of the shaft. (a) small (b) medium (c) large 143. The length cf air gap to be provided in ______ is primarily determined by power factor consideration. (a) d.c. series motor . (b) d.c. shunt motor (c) induction motor (d) synchronous motor 144. Electrical machines having a power output upto about 750 W may be called_______machines. (a) small size (b) medium size (c) large size (d) any of the above 145. Electrical machines having power outputs ranging from a few kW upto approximately 250 kW may be classified as (a) small size machines (b) medium size machines (c) large size machines 146. Commercial available medium size machines have a speed range of ______ . (a) 200 to 400 r.p.m. (b) 600 to 1000 r.p.m. (c) 1000 to 1500 r.p.m. (d) 2000 to 2500 r.p.m. 147. The action of electromagnetic machines can be related to which ofthe following basic principles ? (a) Induction (b) Interaction (c) Alignment (d) All of the above Ans: c,c,a,b,d,d


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148. The change in flux linkages can be caused in which ofthe following ways ? (a) The flux is constant with respect to time and is stationary and the coil moves through it (b) The coil is stationary with respect to flux and the flux varies in magnitude with respect to time (c) Both the changes mentioned above occure together, i.e., the coil moves through a time varying field (d) All ofthe above Ans: d 149. is universally used for windings of electrical machines because it is easily workable without any possibility of fracture. (a) Silver (b) Steel (c) Aluminium (d) Copper Ans: d 150. Aluminium when adopted as a conductor material in ______ transformers, decreases the overall cost of the transformer (a) small size (b) medium size (c) large size (d) any of the above size Ans: a 151. Which ofthe following materials is used in the manufacture of resistance grids to be used in the starters of large motors ? (a) Copper (b) Aluminium (c) Steel (d) Cast-iron Ans: d 152. Materials exhibiting zero value of resistivity are known as ______ . (a) conductors (b) semiconductors (c) insulators (d) superconductors Ans: d


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153. ________ has a lowrelative permeability and is used principally in field frames when cost is of primary importance and extra weight is not objectionable. (a) Cast steel (b) Aluminium (c) Soft steel (d) Cast iron Ans: d 154 ______is extensively used for those portions of magnetic circuit which carry steady flux and need superior mechanical properties ? (a) Grey cast-iron (b) Cast steel (c) High carbon steel (d) Stainless steel Ans: b 155. Hot rolled sheets have ______ value of permeability (a) zero (b) low (c) high (d) none ofthe above Ans: b 156. The heated parts of an electrical machine dissipate heat into their surroundings by which of the following modes of heat dissipation ? (a) Conduction (b) Convection (c) Radiation (d) All ofthe above Ans: d 157. The heat dissipated by from a surface depends upon its temperature and its characteristics like colour, roughness etc. (a) conduction (b) convection (c) radiation (d) any ofthe above Ans: c


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158. The mcrease in heat dissipation by air blasts is due to increase in (a) conduction (b) convection (e) radiation (d) any ofthe above Ans: b 159. On which of the following variables heat convected depends ? (a) Power density (b) Temperature difference between heated surface and coolant (c) Thermal resistivity, density, specific heat (d) Gravitational constant (e) All ofthe above Ans: e 160. Which ofthe following methods is used for air cooling of turbo-alternators ? (a) One sided axial ventilation (b) Two sided axial ventilation (c) Multiple inlet system (d) All ofthe above Ans: d 161. Multiple inlet system of air cooling of turbo-alternators can be used for machines of rating upto (a) 10 MW (b) 30 MW (c) 60 MW (d) 150 MW Ans: c 162. Which ofthe following is an advantage of hydrogen cooling ? (a) Increase in efficiency (b) Increase in ratings (c) Increase in life (d) Smaller size of coolers (e) All ofthe above Ans: e


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163. The density of hydrogen is _____ times the density of air. (a) 0.07 (b) 1.5 (c) 2.5 (d) 3.5 Ans: a 164. Hydrogen has a heat transfer co-efficient _____ times that of air (a) 1.5 (b) 2.5 (c) 3.5 (d) 4.5 Ans: a 165. The thermal conductivity of hydrogen is ______ times that of air (a) 2 (b) 3 (c) 5 (d) 7 Ans: d 166. With conventional hydrogen cooling it is possible to increase the rating of a single unit to (a) 50 MW (b) 100 MW (c) 200 MW (d) none ofthe above Ans: c 167. The noise produced by a ______ cooled machine is less as the rotor moves in a medium of smaller density. (a) air (b) hydrogen (c) either (a) or (b) (d) none of the above Ans: b


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168 cooling is the process of dissipating the armature and field winding losses to a cooling medium circulating within the winding insulation wall (a) Direct (b) Indirect (c) Conventional (d) Any ofthe above Ans: a 169. Machines cooled by direct cooling method may be called (a) "supercharged" (b) "inner cooled" (c) "conductor cooled" (d) any of the above Ans: d 170. In direct cooled system using hydrogen both stator and rotor conductors are made (a) solid (b) hollow (c) perforated (d) any ofthe above Ans: b 171. With direct water cooling it is possible to have ratings of about (a) 200 MW (b) 300 MW (c) 400 MW (d) 600 MW Ans: d 172. The resistivity of water should not be less than (a) 10 Q m (b) 100 Q m (c) 1000 Q m (d) 2000 Q m Ans: d


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173 Direct water cooling of rotor winding presents (a) no mechanical difficulties (b) lesser mechanical difficulties (c) greater mechanical difficulties (d) none of the above Ans: c 174. The time taken by the machine to attain 0.632 of its final steady temperature rise is called (a) heating time constant (b) cooling time constant (c) either (a) or (b) Ans: a 175. In self cooled motors the cooling time constant is about ______ than the heating time constant because cooling conditions are worse at standstill. (a) 2 to 3 times greater (b) 3 to 4 times greater (c) 4 to 5 times greater Ans: a 176. By which of the following methods motor rating for variable load drives can be determined ? (a) Method of average losses (b) Equivalent current method (c) Equivalent torque method (d) Equivalent power method (e) All of the above. Ans: e 177. Which of the following methods does not take into account the maximum temperature rise under variable load conditions ? (a) Equivalent power method (b) Equivalent current method (c) Method of average losses (d) Equivalent torque method Ans: c


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178. Which of the following methods is most accurate ? (a) Equivalent current method (b) Equivalent power method (c) Equivalent torque method (d) Method of average losses Ans: a 179. By which of the following methods the temperature rise of windings and other parts may be determined ? (a) Thermometer method (6) Resistance method (c) Embedded temperature detector method (d) Any of the above Ans: d 180. The slot leakage can be calculated by making which of the following assumptions ? (a) The current in the slot conductors is uniformly distributed over their cross-section (b) The leakage path is straight across the slot and around the iron at the bottom (c) The permanence of air paths is only considered. The reluctance of iron paths is assumed as zero (d) All of the above Ans: d 181. The value of exciting or magnetizing current depends upon which of the following factors ? (a) Total m.m.f. required (b) The number of turns in the exciting winding (c) The way in which the winding is distributed (d) All of the above Ans: d 182. Tractive magnets are operated from (a) a.c. supply (b) d.c. supply (c) either a.c. or d.c. supply (d) none of the above Ans: c


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183 electromagnets generally function as holding magnets. (a) Tractive (b) Portative (c) Either of the above (d) None of the above Ans: b 184. Which of the following is the commonly used type of electromagnets ? (a) Flat-faced armature type (b) Horse shoe type (c) Flat-faced plunger type (d) All of the above Ans: d 185are used for construction of core of electromagnets. (a) Soft magnetic materials (b) Hard magnetic materials (c) Either (a) or (b) (d) None of the above Ans: a 186. The design of electromagnets is based upon which of the following fundamental equations ? (a) Force equation (b) Magnetic circuit equation (c) Heating equation (d) Voltage equation (e) All of the above Ans: e 187. When the two coil sides forming a coil are spaced exactly one pole pitch apart they are said to be of (a) short pitch (b) full pitch (c) either of the above (d) none of the above Ans: b


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188 are always double layer type. (a) Closed windings (b) Open windings (c) Either of the above (d) None of the above Ans: a 189. The distance between the starts of two consecutive coils measured in terms of coil sides is called (a) front pitch (b) winding pitch (c) commutator pitch (d) back pitch Ans: b 190. The winding where dummy coils are used is sometimes called (a) duplex winding (b) triplex winding (c) forced winding Ans: c 191. Dummy coil should not be used in (a) small machines (b) large machines (c) either (a) or (b) Ans: b 192. Power transformers have rating (a) equal to 50 kVA (6) equal to 100 kVA (c) above 200 kVA (d) none of the above Ans: c 193. Power transformers should be designed to have maximum efficiency (a) at one-fourth load (b) at one-half load (c) at or near full load (d) any of the above Ans: c


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194. In transformers using hot rolled steel, the cross-section of the yoke is made about _____ greater than that of the core (a) 5 percent (b) 10 percent (c) 15 percent (d) 30 percent (e) none of the above Ans: c 195. Yokes with rectangular cross-section are used for (a) small capacity transformers (b) medium capacity transformers (c) large capacity transformers (d) any of the above Ans: a 196. The cold rolled grain oriented steel has ______ permeability in the direction of the grain orientation. (a) minimum (b) maximum (c) nil (d) none of the above Ans: b 197. Cylindrical windings using circular conductors, employed in transformers, are (a) single layered (b) double layered (c) multi-layered (d) none of the above Ans: c 198. Helical windings are used in (a) distribution transformers (b) power transformers (c) shell type transformers (d) none of the above Ans: b


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199. Multi-layer helical windings are commonly used in the transformers as high voltage windings (a) upto 20 kV (b) upto 50 kV (c) upto 80 kV (d) for 110 kV and above Ans: d 200. Disc windings are primarily used in (a) short capacity transformers (b) medium capacity transformers (c) high capacity transformers (d) any of the above Ans: c 201. The heat dissipating capability of transformers of ratings higher than 30 kVA in increased by providing which of the following ? (a) Corrugations (b) Fins (c) Tubes (d) Radiator tanks (e) All of the above Ans: e 202. Transformers with a capacity of upto _____ have a cooling radiator system with natural cooling (a) 2 MVA (b) 5 MVA (c) 7.5 MVA (d) 10 MVA Ans: d 203. The forced oil and air circulation method is usual one for transformers of capacities (a) upto 5 MVA (b) upto 10 MVA (c) upto 20 MVA (d) 30 MVA upwards Ans: d


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204. The flash point of transformer oil should be higher than (a) 40°C (b) 60°C (c) 80°C (d) 104°C Ans: d 205. The voltage control in electric supply networks in required on account of which of the following reasons ? (a) Adjustment of voltage at consumers premises within statutory limits (b) Control of active and reactive power (c) Adjustment of short period daily and seasonal voltage variations in accordance with variations of load (d) All of the above Ans: d 206. D.C. windings are (a) sometimes 2-layer type (b) never 2-layer type (c) always 2-layer type (d) none of the above Ans: c 207. The usual values of maximum flux densities for distribution transformers using hot rolled silicon steel are (a) 0.5 to 0.8 Wb/m2 (b) 0.8 to 1.0 Wb/m2 (c) 1.1 to 1.35 Wb/m2 (d) 1.4 to 1.8 Wb/m2 Ans: c 208. For 275 kV transformers, using cold rolled grain oriented steel, which of the following values of flux density may be used? (a) 1.0 Wb/m2 (b) 1.1 Wb/m2 (c) 1.3 Wb/m2 (d) 1.6 Wb/m2 (e) None of the above Ans: d


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209. For large power transformers, self oil cooled type or air blast type which of the following values of current density may be used ? (a) 1.0 to 1.2 A/mm2 (b) 1.5 to 2.0 A/mm2 (c) 2.2 to 3.2 A/mm2 (d) 3.2 to 4.2 A/mm2 Ans: c 210. A current density of _____ is ilsed for large power transformers with forced circulation of oil or with water cooling coils (a) 1.5 to 2.5 A/mm2 (b) 3.5 to 4.5 A/mm2 (c) 4.0 to 5.0 A/mm2 (d) 5.4 to 6.2 A/mm2 Ans: d 211. The high voltage winding is usually which of the following type ? (a) Cylindrical winding with circular conductors (b) Cross-over winding with either circular or small rectangular conductors (c) Continuous disc type winding with rectangular conductors (d) All of the above types Ans: d 212. Which of the following is the basic consideration in the design of insulation ? (a) Electrical considerations (b) Mechanical considerations (c) Thermal considerations (d) All of the above Ans: d 213. A practical formula for determining the thickness of insulation between low voltage and high voltage windings is (a) 1 + 0.2 kVmm (6) 2 + 0.5 kVmm (c) 4 + 0.7 kV mm (d) 5 + 0.9 kV mm Ans: d


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214. The insulation between windings and grounded core and the insulation between the windings of the same phase is called (a) minor insulation (b) major insulation (c) either of the above (d) none of the above Ans: b 215. The cylindrical windings using circular conductors are used for current rating of (a) upto 20 A (b) upto 40 A (c) upto 60 A (d) upto 80 A Ans: d 216. The surge phenomenon is particularly important in case of (a) low voltage transformers (b) medium voltage transformers (c) high voltage transformers (d) any of the above Ans: c 217. Which of the following in an application of D.C. motors? (a) Traction (b) Drives for process industry (c) Battery driven vehicles (d) Automatic control (e) All of the above Ans: e 218. D.C. servomotors are used in (a) purely D.C. control systems (6) purely AC. control systems (c) both D.C. and AC. control systems (d) none of the above Ans: a


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219. The stator of a D.C. machine comprises of (a) main poles (6) interpoles (c) frame (d) all of the above Ans: d 220. The laminations of the armature of a D.C. machine are usually _____ thick. (a) 0.1 to 0.2 mm (b) 0.2 to 0.3 mm (c) 0.3 to 0.4 mm (d) 0.4 to 0.5 mm Ans: d 221. is usually used for brush rockers (a) Mild steel (b) Copper (c) Aluminium (d) Cast-iron Ans: d 222. ______ brushes are fragile and cause excessive wear of commutator, (a) Natural graphite (b) Hard carbon (c) Electro graphitic Ans: a 223. Which of the following brushes can be used for high values of current density ? (a) Metal graphite brushes (b) Hard carbon brushes (c) Electro-graphitic brushes Ans: a 224 ________ is the common method of applying brushes to the commntator. (a) Radial (b) Trailing (c) Reaction (d) All of the above Ans: a


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225 Which of the following problem arises in D.C. motors which are fed from thyristor bridge circuits ? (a) Increased I R losses (b) Increased core losses (c) Poor commutation (d) Change in motor parameters (e) All of the above Ans: e 226. The weight of copper of both armature and field windings decreases with _____ in number of poles. (a) increase (b) decrease (c) either of the above (d) none of the above Ans: a 227. In a D.C. machine the number of brush arms is _____ the number of poles. (a) less than (b) equal to (c) greater than (d) none of the above Ans: b 228. In a D.C. machine the current per brush arm should not be more than (a) 100 A (6) 200 A (e) 300 A (d) 400 A Ans: d 229. In a D.C. machine, the value of peripheral speed should not, normally, exceed (a) 10 m/s (b) 20 m/s (c) 30 m/s (d) 40 m/s Ans: c


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230. In D.C. machines the width of the duct is usually (a) 4 mm (b) 6 mm (c) 8 mm (d) 10 mm Ans: d 231. D.C. machines designed with a large value of air gap length have (a) worst, ventilation (b) poor ventilation (c) better ventilation (d) none of the above Ans: c 232. In D.C. machines, ir order to prevent excessive distortion of field form by the armature reaction, the field mmf must be made (a) equal to that of armature mmf (b) less in comparison with the arma¬ture mmf (c) large in comparison with the arma-ture mmf (d) none of the above Ans: c 233 In D.C. machines, the ____ in field mmf results in increase in size and cost of machines. (a) increase (b) decrease (c) either of the above (d) none of the above Ans: a 234 The operation of a D.C. machine with large air gap lengths is comparatively (a) quiet (b) noisy (c) either of the above (d) none of the above Ans: a


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235. Which of the following methods may be adopted to reduce the effects of armature reaction ? (a) Increase in length of air gap at pole tips (b) Increasing reluctance of pole tips (c) Compensating windings (d) Interpoles (e) All of the above Ans: e 236. In D.C. machines the usual limit of slot pitch is (a) between 5 to 10 mm (b) between 10 to 15 mm (c) between 15 to 20 mm (d) between 25 to 35 mm Ans: d 237. In D.C. machines the number of slots per pole usually lies (a) between 2 to 4 (b) between 6 to 8 (c) between 9 to 16 (d) between 20 to 30 Ans: c

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MCQ QUESTION BANK Design of DC Machine and Transformer 1